Vat alkaline dyeing processes for use with textile fibers are known; see, for example, British Patent 534,085, International Patent Application WO96/04420, and U.S. Pat. No. 3,353,900. A vat neutral dyeing process has been disclosed for use with polypropylene fibers, American Dyestuff Reporter, March 1997, pp. 15-18, 66.
Vat acid dyeing processes have been disclosed in British Patents 709,150 (in which, however, the reduced dye is reoxidized before the dyeing step), 712,418 and 1,383,451, U.S. Pat. Nos. 2,627,449 and 3,527,556, Applications of Leuco Vat Acid Dispersion on Polyester (S. N. Chevli, Master's Thesis, University of Leeds, UK, 1997), and American Dyestuff Reporter, Sep. 17, 1951, pp. 585-596.
The use of sulfinic acid reducing agents in alkaline vat dyeing has been disclosed in U.S. Pat. No. 6,007,587 and British Patent 1,430,179 and, in post-dyeing reduction clearing, in International Patent Application WO98/03725. The use of sodium formaldehyde sulfoxylate with vat dyes in printing and its instability in dilute acids have been disclosed in The Merck Index, Eighth Edition, Merck & Co., Inc., 1968, p. 959 and in Rongolit® C trade literature from BASF (TI/T 5952e, Febuary 1997).
However, none of these processes provides adequate depth of shade, washfastness, or stain-resistance, and an improved dyeing method is still needed.